1. Field of the Invention
This invention relates to an operation method and an operation control device of an internal combustion engine for a vehicle that temporarily stops an engine, in which an operation of the internal combustion engine is stopped by control determination by a vehicle operation control device during the vehicle operation. More particularly, the invention relates to an improvement of a control of a vehicle operation control device.
2. Description of the Related Art
In recent years, since air environmental conservation has been demanded, an exhaust system of an internal combustion engine for a vehicle such as an automobile is provided with a catalytic converter including an exhaust gas purification catalyst such as three-way catalyst or a lean NOx catalyst which treat HC, CO, and NOx that are generated by an operation of the engine into harmless H2O, CO2, and N2. In order to activate and effectively operate the exhaust gas purification catalyst in the catalytic converter, it is necessary to heat the exhaust gas purification catalyst to a significantly high temperature of approximately 700° C. The high temperature state is achieved by an introduction of high-temperature exhaust gas which is exhausted from the internal combustion engine into the catalytic converter resulting in heat-up of the catalyst and by heat-up of the catalyst due to heat generated by oxidation of unburned components such as HC and CO in catalyst layers.
Moreover, in order to deal with the importance of fuel resource saving as well as the air environmental conservation, an economy running car and a hybrid car have been in the limelight. In a hybrid car, the vehicle is driven by the combination of driving by the internal combustion engine and driving by the motor based on a control determination by the vehicle operation control device based on the vehicle operating state during a vehicle operation. In an economy running car, the internal combustion engine is temporarily stopped by the control determination by the vehicle operation control device while the vehicle is temporarily stopped during the vehicle operation. In addition, with a hybrid car in particular, it is possible to change the control of the internal combustion engine with a considerable degree of freedom, by the combination use of the internal combustion engine and the motor. Focusing on this point, it is proposed in the Japanese Patent Laid-Open Publication 6-165308 that a speed of the internal combustion engine and the load thereof are controlled according to a schedule based on the catalytic converter temperature.
Further, with the development of micro computers in recent years, vehicle operation control devices including a micro computer have been increasingly used for controlling the operation of an internal combustion engine of vehicles. Accompanied by this, a fuel cut, in which fuel supply to the internal combustion engine is cut during an operation like a vehicle deceleration when the internal combustion engine is not required to produce power, has been increasingly used. However, if the fuel cut as above is performed, a large amount of oxygen is flown into the catalytic converter, even if the catalyst in the catalytic converter is in a high-temperature activated state. For this reason, there is a possibility that the catalyst is deteriorated by oxygen. In order to solve this problem, the Japanese Patent Laid-Open Publication 2001-59444 discloses prohibition of the fuel cut when the catalyst temperature is high.
In the mean time, the catalyst in the catalytic converter is heated by the high temperature exhaust gas which is introduced into the catalytic converter from the internal combustion engine as mentioned above. However, there are some cases where the catalyst is cooled by the exhaust gas which passes and flows into the catalytic converter. In other words, the catalyst in the catalytic converter is heated by the exhaust gas while the exhaust gas flow through the catalytic converter is maintained. In addition, if the catalyst temperature becomes too high, the exhaust gas flow has deprived the catalyst of the heat, thus suppressing the increase in the catalyst temperature. The catalyst temperature is maintained at an appropriate temperature under such thermal equilibrium. Therefore, unlike the case where the fuel cut is performed in which the fuel injection into the internal combustion engine is stopped and at least some of the intake air is supplied to the internal combustion engine, when the internal combustion engine is stopped, the exhaust gas flow passing across the catalyst layers is completely stopped. Accordingly, the catalyst is not deprived of the heat by the exhaust gas.
Moreover, all of the heat which is generated by the unburned components which has been already carried into the catalytic converter at the moment when the engine stops stays in the catalytic converter. For this reason, if the internal combustion engine is stopped and the exhaust gas flow which passes the catalytic converter is stopped, the catalyst temperature in the catalytic converter is increased for a while.
In this case, there is no problem if the catalyst temperature during the engine stop does not impair the durability of the catalyst after the temperature is increased by the aforementioned phenomenon. However, if the catalyst temperature during the engine stop exceeds that temperature, the durability of the catalyst may be impaired by the temperature increase. This is a serious issue especially in a hybrid car and an economy running car since they involve frequent stops of the internal combustion engine.